A light or radiation detector has a semiconductor for generating electric charges in response to incident light or radiation, and a support substrate for supporting the semiconductor laminated thereon. Radiation (e.g. X-ray) detectors include an “indirect conversion type” detector which once generates light upon incidence of radiation (e.g. X rays) and generates electric charges from the light, thus detecting the radiation by converting the radiation indirectly into the electric charges, and a “direct conversion type” detector which generates electric charges upon incidence of radiation, thus detecting the radiation by converting the radiation directly into the electric charges. In the “direct conversion type” detector, the semiconductor which generates electric charges is a radiation sensitive semiconductor.
As this radiation sensitive semiconductor, films of CdTe, ZnTe, HgI2, PbI2, PbO, BiI3, TlBr, Se, Si, GaAs, InP and so on formed by physical vapor deposition (PVD) are used or considered. As film forming methods for high sensitivity materials such as CdTe, sputtering, CVD, sublimation method and chemical deposition method are known, and polycrystal films are obtained by all these methods. The detection characteristics of polycrystal films with respect to light or radiation are greatly dependent on crystalline morphology of the films, and thus greatly dependent on film forming conditions.
Incidentally, physical vapor deposition includes a method called “proximity sublimation method”. This proximity sublimation method is a method in which a deposition source and a supporting substrate which is an object for having a semiconductor formed on its surface are placed close to each other, and the semiconductor is formed of a sublimate of the source on the surface of the supporting substrate. Since the source is in proximity in this proximity sublimation method, a semiconductor of large area can be formed relatively easily.
However, with semiconductor films formed by physical vapor deposition represented by these methods, a film formed adjacent a substrate interface at an early stage from a surface layer of the source has poor crystallinity. It has been confirmed experimentally that, with a CdZnTe film formed by the proximity sublimation method in particular, the film formed on the substrate at an early stage from a surface layer of the source has poor crystallinity, becoming a cause of deteriorating detection characteristic.
So, there is a technique of forming a semiconductor film on the surface of the substrate after cutting the film formed by vapor deposition at an early stage, by providing a shielding device between the deposition source and the film forming surface of the substrate (see Nonpatent Document 1, for example). According to this technique, since impurities are mixed in the source in an initial state, the shield is provided at an early stage to cut the defective film to be formed by vapor deposition at the early stages. Therefore, by removing the shield subsequently, the semiconductor film formed after the removal has high quality, thereby improving detection characteristics.
[Nonpatent Document 1]
“Handbook of Thin Film Formation/Evaluation and its Application Technology” by Yasuo Nakai, Fuji Technosystem, p. 250.